rusty,
Here's the Tarheel Hal with the engine in the previous post:
https://www.youtube.com/watch?v=ZzPiCEolZ8s
That is a 55 with a 22x10 prop. It needed 24 ounces of bb's and epoxy poured in just behind the firewall to balance. It flies just fine and is far from anemic!
My new Tarheel Hal, renamed the Missouri Fury, has a G-62 and needed no weight and balanced just fine. Hasn't flown yet but will as soon as the weather around here stabilizes.
The 55RA will do fine in the Spitfire and will simplify your installation. Also, I think it would be cheaper than buying an engine then spending another $130 on a muffler.